Question

What is rocket propulsion? derive its formula

Answer 1

Principle of rocket propulsion is based on the following two laws:

 1. Newton's third law:
 According to Newton's third law, whenever a body exerts a force on another body, the other body exerts an equal and opposite force on the rst body. Thus, forces always occur in pair and never alone.
 In rocket propulsion, Action force: The rocket expels a jet of hot gases from its tail.
 Reaction force: The jet of hot gases exerts a force on the rocket, propelling it forward.

2. Law of Conservation of momentum: Conservation of momentum states that the momentum of the body remains conserved if no external force is applied on it. In rocket propulsion,
 The hot gases acquire momentum in the backward direction and the rocket acquires an equal amount of momentum in the forward direction.
 Let the rocket has an initial speed u . Let a mass Δm be released from it at a speed u and the velocity of the rocket change to v.

Force on rocket due to mass Δm release is,
 F12 = M(v – u )/Δt

The force on the released mass by the rocket is,
 F21 = Δmu/Δt
 By third law of motion,
 F12 = -F21
=> M(v – u )/Δt = -Δmu/Δt
 => M Δv = -Δm u 



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